Guerrilla is Spanish, and it means "little war" — a diminutive of guerra, "war," using the same -illa ending that turns a cigarro into a cigarette-sized cigarrillo. The doubled r and the double l are Spanish spelling conventions English kept without adopting the sounds behind them; almost no English speaker rolls the r or gives the ll its Spanish value.
There is a nice loop in the origin. Spanish guerra is itself Germanic, from the same root as English war, borrowed into the Romance languages because Latin's own word, bellum, had worn away. English took the Spanish word in 1809, during the Peninsular War, when Spanish peasants and shepherds were harassing the occupying French. Purists tried to keep it meaning only the style of warfare, arguing the fighter should be a guerrillero; they lost, as purists generally do.
guerrilla (n.) "fighter in an irregular, independent armed force," 1809, from Spanish guerrilla "body of skirmishers, skirmishing warfare," literally "little war," diminutive of guerra "war," from a Germanic source cognate with Old High German werra "strife, conflict, war," from Proto-Germanic *werra- (see war (n.)). Acquired by English during the Peninsular War (1808-1814), when bands of Spanish peasants and shepherds annoyed the occupying French. Purists failed in their attempt to keep this word restricted to "irregular warfare" and prevent it taking on the sense properly belonging to guerrillero "guerrilla fighter." Figurative use by 1861. As an adjective from 1811. also from 1809
guerrilla came in through Later and wider borrowings, and its spelling follows that family's conventions.
How that family writes these sounds
| a rolled or doubled liquid in a Spanish loan | ll, rrguerrilla, armadillo |
What gives it away on the page
| -o/-a endings, ll, rr, ny for ñ | Spanish — much of it from the Americas.tornado, cargo, plaza, armadillo, guerrilla, canyon, mosquito |
Later and wider borrowings in the spelling guide · guess another word's family